Fix backslash in TeX in BVProblemLibrary.jl

[abhro]

Checklist

• Appropriate tests were added
  None applicable
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Fix frac to \frac